It is known to reinforce materials such as plastics materials and epoxy resins, with coarse plant fibres from hemp and flax, for example. It is known to use the cellulose or cellulose fibres which make up the cell walls of the plant cells as an additive to bestow desired properties on a composition, for example, to increase the viscosity of an aqueous medium or emulsion. It is also known to use microfibrilar cellulose as an additive to modify the physical or chemical properties of a material. Microfibrilar cellulose is typically produced from plant cell walls, which comprise mostly cellulose, hemicellulose and pectin, which are broken down into their constituent cellulose fibres, and then further broken down into the individual microfibrils.
Typically, cellulose additives are broken down to microfibrils because of the high viscosity which can be achieved per unit weight of cellulose, which arises in part from the ability of cellulose microfibrils to form a lattice-type network within the medium to which they are added. For example, WO 2011/039423 (UPM-Kymmene Corporation) discusses the use of microfibrillar cellulose as an additive for concrete where the diameter of the microfibrils is preferably less than 1 μm.
Each stage of breaking down the cellulose involves a number of processes, including chemical treatment, homogenisation through either high shear mixing or through grinding, filtration and pressing. The larger the number of processes required, the more energy required to produce the end product.
One problem with microfibrillar cellulose is that it usually forms a gel or other solid at relatively low proportion by weight of solids. For example, microfibrillar cellulose of 5% by weight of solids is likely to form a solid. Therefore, in order to obtain a higher concentration, to allow a relatively small amount to be used as an additive in a large volume of media, for example, the microfibrillar cellulose must be compressed to force more of the water out of it. However, once dried, the cellulose microfibrils typically form inter-fibril bonds that must be broken if the cellulose microfibrils are to be suspended in aqueous media. In many compositions, resuspension is not possible if the composition is compressed too much (to 10% by weight of solids, for example). Accordingly, such compositions are often bulky and so inefficient and expensive to store or transport.
One method of overcoming this problem is disclosed in U.S. Pat. No. 5,964,983 (Dinand et al.) which discloses a suspension of microfibrillar cellulose which can be recreated after it has been dehydrated. However, rehydration of the disclosed microfibrillar cellulose requires the use of high shear mixing, where aggregates of microfibrils are effectively broken down again. Therefore, although the disclosure of Dinand et al. potentially provides microfibrillar cellulose for use as an additive that may be rehydrated, the cellulose high shear mixing is energetically expensive and cannot be used with emulsion paints or other composites which must not be subjected to high shear forces.
U.S. Pat. No. 4,831,127 (Weibel) discloses a method of separating out the cellulose and hemicellulose components from parenchymal cells whereby the cellulose component is in the form of platelets. The composition is made by heating the parenchymal cells to high temperatures (approximately 180° C.) and high pressures (approximately 300 pounds per square inch) whilst exposing the cells to extreme pH (either very high pH or very low pH) to remove as much hemicellulose as possible. The composition of Weibel following high shear homogenisation forms a stable homogeneous suspension having high viscosity and weak thixotropic character.
However, the cellulose compositions so formed are very difficult to rehydrate, due to the formation of internal hydrogen bonds between cellulose layers or fibres of the cellulose platelets during the dehydration process. Therefore, the cellulose composition of Weibel is not capable of being dehydrated to a useful concentration whilst still be able to be subsequently fully rehydrated.
Therefore, it is one object of the present invention to provide a cellulose composition and a method of producing a cellulose composition suitable for use as an additive that may be rehydrated using low shear forces.
It is an additional object of the invention to provide a cellulose composition and a method of producing a cellulose composition that has a relatively high viscosity in comparison with known cellulose compositions to allow smaller amounts of the composition to be added to a medium to achieve a given viscosity.
Many water-based materials, such as paints, for example, require separate additives to be added to thicken the water, to interact with the polymer, to prevent the pigment particles from settling out. The necessity to add many separate additives increases the cost of the resultant paint.
Accordingly, it is a further object of the invention to provide a cellulose composition that can perform two or more roles within a material to which it is added.